It is known to protect various thermoplastic resin compositions against fungal or bacterial attack by incorporating a microbiocide therein to prevent the deterioration of articles formed from the resin compositions. Microbiocides inhibit growth of bacteria or fungi by acting upon the cell wall or upon cell proteins, e.g., by attacking disulfide bonds. In order for the microbiocide to be effective in the resin composition, it is necessary that it be compatible with the components of the composition and be uniformly dispersible in the resin composition. The biocide must be carried by the resin composition in a manner that it remains biologically active against microorganisms, and, in particular, must be available at the surfaces, including internal pore surfaces. Incorporation of biocides in resin compositions is generally effective only in compositions in which the biocide is able to slowly migrate to the surfaces. In some cases, the biocide migrates slowly through amorphous regions of the polymer. In other cases, biocide migration is facilitated by plasticizers which are included along with the polymeric resins in end use resin compositions. As the biocide at the surfaces is used up through action against microorganisms, additional biocide migrates to the surfaces. Although biocides may be highly toxic chemicals, their low concentration in the end use product and their retention by the resin composition ensure that biocides in the end use product pose no hazard to humans or animals.
Biocides must be available in a form that is readily dispersible into the formulation mix from which the resin composition is fabricated. The powdered or crystalline form in which many useful biocides are commercially available are readily dispersible; however, at the site of mixing, powdered or crystalline biocides pose a substantial environmental and health hazard if powder or crystal fines are dispersed into the atmosphere. Furthermore, powder, or powdered fines, if dispersed into the atmosphere, represent a potential explosive hazard.
Recognizing the toxicity problem of biocides in powder or crystalline form, U.S. Pat. No. Re. 29,409, the teachings of which are incorporated herein by reference, teaches dissolving biocides in a liquid solvent which may be added to the formulation mixture from which the end use resin composition is fabricated. Although liquid dispersions may be safely used at the site of preparing an end use resin composition, careless use or disposal of the liquid may still pose environmental and health hazards.
U.S. Pat. No. 4,086,297 issued Apr. 25, 1978 to Rei, et al., the teachings of which are incorporated herein by reference, describes solid thermoplastic biocide resin concentrates containing immobilized biocides. These solid biocide resin concentrates contain relatively high concentrations of biocides and may be added to the formulation mixture from which the end use resin composition is prepared in an amount sufficient to provide the desired end use biocide concentration. The solid biocide resin concentrates, which are typically provided in the form of small pellets, can be handled freely, posing substantially no health or environmental threat. Such pellets are even safe for direct skin contact. Although biocides are sufficiently immobilized and inactive in the solid biocide resin concentrates in softer end use resin compositions, the low concentration biocides at the surface have biological activity, and gradual and continuous migration to surfaces ensures continuous biological activity. Where practical, solid biocide resin concentrates represents a preferred manner of providing biocides to producers of end use thermoplastic products.
In order that solid thermoplastic resin compositions be a practical means of providing biocides to the site of preparation of the end use thermoplastic resin composition, the biocide must be stably incorporated into the solid resin composition at concentrations substantially above end use concentrations. Generally, the concentration of the biocide in the solid biocide resin concentrate must be between about 20 and about 500 times the concentration of the biocide in the end use resin composition, and typically the concentration of the biocide in the solid biocide resin concentrate is about 100 times the end use concentration, enabling the solid biocide resin concentrate to be added to the non-fabricated resin compound at one part per hundred of the total amount of thermoplastic resin added to the non-fabricated resin compound. As a practical matter, the concentration of biocide in the solid biocide resin concentrate cannot be too low, or excessive amounts of thermoplastic resin must be pre-processed to contain a sufficient amount of biocide. Also as a practical matter, the concentration of the biocide in the solid biocide resin concentrate should not be excessively high, in which case, only a very small amount of the pre-processed solid biocide resin concentrate would be added to the non-fabricated compound, posing substantial problems with respect to obtaining a homogenous blend.
It has been found that certain biocides are not stably incorporated into particular resins at the concentrations required for providing a preprocessed solid biocide resin concentrate. In particular, an important and widely used biocide, 10,10'-oxybisphenoxarsine, (OBPA) is not sufficiently compatible in polyethylene resin for a practical polyethylene/OBPA solid biocide resin concentrate to be produced. For most end uses, in order to provide adequate biocidal protection for the end use product, OBPA is present at about 0.05 weight percent of thermoplastic resins, requiring that a solid biocide resin concentrate contain at least about 1.0 wt % (at a 20.times.concentration) and preferably about 5 wt % (at a 100.times.concentration). In fact, OBPA an only be stably incorporated in solid polyethylene resin concentrate at up to about 0.2 wt. percent. At higher OBPA concentrations, OBPA blooms or spews to the surface of polyethylene. Such a biocide bloom represents a health hazard to anyone handling the solid biocide resin concentrate.
A 0.2 weight percent admixture of OBPA with polyethylene resin would require a one part per four (total) admixture of solid biocide resin concentrate with additional polyethylene resin in the end use resin compound formulation, a commercially unacceptable requirement. This would require pre-processing large quantities of thermoplastic polyethylene resin, and there is little commercial advantage in doing so.
OBPA is stably incorporated at desirable 5 wt. percent levels in other polymeric resins, such as polystyrene and polypropylene; however, these resins differ in physical properties from polyethylene. OBPA-containing resins of polypropylene, polystyrene and other previously prepared solid biocide resin concentrates represent an adulteration to a polyethylene end use product and tend to significantly affect the physical properties of the end use product. Also, due to their respective melt indicies, these polymers do not mix easily with polyethylene. Accordingly, such solid biocide resin concentrates may be unsuitable for use in a resin compound from which is fabricated an end use resin composition having particular properties. Even where end use properties are not highly critical, the manufacturer of the end use resin composition may be reluctant to adulterate the polyethylene with the foreign polymer of the solid biocide resin concentrate, which typically represents about 1% of the total thermoplastic polymeric resin.
It is desired to have new solid thermoplastic biocide resin concentrates in which biocides are stably immobilized at commercially desirable concentrations. In particular, it is desired to have sold biocide resin concentrates in which OBPA is stably immobilized at commercially acceptable concentrations, which solid biocide resin concentrates include a resin component or combination of components which more closely resemble and are more closely compatible with polyethylene in end use resin compositions than are presently available OBPA-containing solid resin compositions. It is also desirable to provide solid biocide resin concentrates which can be mixed with a variety of different polymers.